This invention in general relates to optical communications and in particular to amplifiers for use in increasing signal strength in optical fiber based communications systems.
In optical communications, information is encoded by modulating according to well-known schemes the optical power from a source to produce an optical signal. Afterward, the signal, or signals where some form of multiplexing is in use, is introduced into an optical fiber transmission line and, after travelling along the line for some distance, is decoded at the receiving end. As the signal travels along the transmission line, it suffers losses due to well-known attenuation effects associated with the optical fiber characteristics. The amount of signal attenuation allowable is related to the initial signal power, usually considered fixed for a given source, and the minimum power required at the receiver for accurate signal decoding. Consequently, the transmission distance is limited by the ratio of the input signal power to the required receiver power. For transmission over greater distances, the initial signal must be amplified without any significant loss of information through distortion or amplification with signal reconstruction is required, a process performed by a device ususally referred to as a repeater rather than an amplifier which raises signal strength without reconstruction. Thus, amplifiers are an important element in optical communication systems and this holds regardless of the particular modulation format utilized.
Early repeaters were electro-optical devices which converted optical signals to their electrical equivalents, increased their amplitude, and then reconverted them back to optical form for retransmission along the trunk. Although these are still in use and are perfectly suitable for particular applications, they do suffer from obvious disadvantages which include the need for separate power sources and some means of conveniently coupling to and from the transmission line. In addition, multiplexed signals require some means of separation by wavelength followed by a separate amplification stage for each signal separated after which some means of recombination for retransmission is necessary.
Purely optical amplifiers without the disadvantages of the electro-optical repeaters are known in the art as, for example, that shown and described in U.S. Pat. No. 3,894,857 issued to Teiji Uchida et al on July 15, 1975 for a "Process for Exchanging Alkali Ions for Thallium Ions in a Glass Fiber". Here, the amplifier is a length of optical glass fiber with a gradient index for waveguiding action and laser active substance for amplification purposes. The fiber can be coiled around a source or the source can be arranged along a length of the fiber to introduce pumping energy to the active region of the fiber to effect amplification of signals guided within its core. Other examples of optical amplifiers are given by Elias Snitzer in his paper entitled "Fiber Lasers and Dispersion in Fibers" from Proceedings of the First European Electro-optics Markets and Technology Conference, pp 374-378, Geneva (Sept. 1972) published by JPC Science and Technology Press.
Thus, optical amplifiers are known and are attractive for their simplicity. However, optical amplifiers are not without difficulty. Among these are the need for long amplification length and/or great pump power for sensible signal gain levels and precise control over atomic population levels is required for uniform levels of amplification.
Consequently, there is still a need for improved optical amplifiers, and it is a primary object of this invention to provide an optical amplifier of simple construction with reasonable gain.
It is another object of this invention to provide a multiple pass optical amplifier of relatively short length.
It is yet another object of this invention to provide an optical amplifier of uniform gain with ease of gain control.
It is yet another object of this invention to reduce the noise associated with spontaneous emission to only those wavelength intervals in which signal amplification occurs.
Other objects of the invention will in part be obvious and will in part appear hereinafter. The invention, accordingly, comprises the apparatus possessing the construction, combination of elements and arrangement of parts which are exemplified in the following detailed disclosure.